In recent years, the high-density integration of electronic parts and high-density mounting etc. have been advancing along with demand for highly advanced functions of electronic devices, and printed wiring board for high-density mounting used therein is also becoming down-sized and highly densified.
Among them, a metal layer-coated resin film substrate is widely used as a substrate for mounting of driving semiconductors for displaying an image on a liquid crystal display or as a substrate used in an operating part requiring flexibility. In recent years, COF (chip-on-film) attracts attention as a means for mounting driver IC chips for liquid crystal display. It is said that COF as compared with the conventional major mounting technique TCP (tape-carrier-package) enables fine pitch mounting and enables down-sizing of driver IC and cost reduction. In COF, there is a strong demand in recent years for high-definition and fine pitch electronic circuits along with high definition of recent liquid crystal displays and down-sizing of liquid crystal driving ICs.
In formation of these fine pitch wirings, the “subtractive method” and “semi-additive method” are known as metal pattern forming methods useful in conventional electroconductive patterns, particularly in printed-wiring boards. The subtractive method is a method wherein a photosensitive layer sensitive to irradiation with active ray is arranged on a metal layer formed on an insulating resin film, and this photosensitive layer is subjected to imagewise exposure and developed to form a resist image, and then the metal is etched to form a metal pattern, and finally the resist is removed. The metal-coated resin substrate used in the subtractive method is formed by arranging a resin varnish layer on a metal foil and solidifying the resin varnish or by arranging a thermoplastic layer on an insulating resin film and laminating a metal foil thereon, or by arranging a feed layer on the surface of an insulating resin film by some methods and passing a current through the feed layer thereby conducting electroplating. The method used in forming the feed layer may be plating, sputtering, vapor deposition and lamination of a thin metal foil.
On the other hand, the semi-additive method is a method wherein a feed layer is arranged on the surface of an insulating resin film by some methods, a photosensitive layer sensitive to irradiation with active ray is arranged on the feed layer, this photosensitive layer is subjected to imagewise exposure and developed to form a resist image, the feed layer is electrified to effect electroplating to form a metal wiring on the non-resist portion and then the feed layer in the non-metal wiring part is etched to form a metal pattern. The method used in forming the feed layer may be plating, sputtering, vapor deposition and lamination of a thin metal foil. However, a metal-coated resin substrate having a resin varnish layer formed on a metal foil, or a laminate having a thermoplastic layer formed between a metal foil and an insulating resin film, has a problem that when fine wirings are to be formed, fine wirings cannot be formed due to uneven surface attributable to the metal foil. On the other hand, when the unevenness of the copper foil is reduced, there is a problem that sufficient adhesion between the metal layer and the insulating resin film cannot be attained.
In the method of arranging a feed layer by metal sputtering or vapor deposition, a metal layer is arranged on a smooth surface and thus fine wirings can be easily formed, but the adhesion between a metal and an insulating resin film is not sufficient, and production thereof requires large vacuum facilities, the operation is complicated, the forming speed is low, so the yield is low and the costs are high. Since a hole for the connection between circuits of both sides is made after formation of a metal layer on the whole surface, a connection in the hole by plating is separately necessary, and a metal layer by plating should be formed on the previously formed metal layer, thereby making the metal layer thicker than necessary and thus necessitating long etching time in forming the circuit.
As described above, when a wiring pattern is formed on a substrate or an insulating resin film, the adhesion between the insulating resin film and the wiring pattern becomes problematic. For example, when a polyimide varnish layer that is an insulating resin is formed on a copper foil, from which a metal layer-coated resin film is obtained by heat reaction, its adhesion is attributable to the adhesion between the copper foil layer and the polyimide varnish layer. However, when the surface of the copper foil is made uneven for increasing adhesion by an anchor effect, it is necessary that as the distance between wirings decreases, the unevenness is decreased to such an extent that the shape of wirings is not influenced, so there is a problem that sufficient adhesion cannot be achieved. On the other hand, the method of forming copper on polyimide by sputtering does not attain sufficient adhesion and the film making speed is low, and thus there is a problem that the cost is increased.
To solve this problem, there is a proposed method wherein the surface of a substrate is modified by grafting with a radically polymerizable compound, thereby minimizing the unevenness of the substrate and facilitating the step of processing the substrate (for example, see Japanese Patent Application Laid-Open (JP-A) No. 58-196238), but this method requires expensive apparatuses (γ-ray generator, electron beam generator). The substrate used is a usual commercial plastic substrate, and thus the graft polymer is not sufficiently generated to such an extent that a conductive material is strongly adhered thereto, so there is a problem that the adhesion between a substrate and a conductive layer does not reach practical strength. As one means of forming an electroconductive layer, a method wherein a surface graft polymer whose polymer terminal is fixed on the surface of a substrate is used to accumulate gold nanoparticles at one step has been reported (see, for example, Liz-Marzan, L. M. et al., J. Phys. Chem. Vol. 99, p. 15120 (1995) and Carignano, M. A. et al., Mol. Phys. Vol. 100, p. 2993, (2002)). Under the conditions described therein, however, practically satisfactory interaction is not formed in the phenomenon of particle aggregation by the electrostatic force between the charged polymer and charged particles, so there is further demand for improvement in practical adhesion to conducive materials.
In formation of such surface graft polymers, a step of applying energy to monomer components as the starting material of graft polymers while the monomer components are contacted with the surface of a substrate is necessary, but there is a problem that it is difficult to maintain uniformity in the step, such as uniform contact and uniform energy application.